You can use an aplication like mkvtoolnix on your pc and attach the sub/idx file to the video file and make one single video file with hard sub and after that enjoy watching the video with hard sub on your android device. Hope this will help you
You can use VLC to view videos with subtitles. Copy the subtitle files on the same folder as the video, then change their names to be the same as the video (don't change their extension).
MoviePlayer and mVideoPlayer both support sub/idx subtitles.